In the field of converting hydraulic energy into electrical or mechanical energy and vice versa, the use of a Francis pump-turbine is known. Such a pump-turbine includes a runner which is movable about a vertical axis and which is rigidly connected to a drive shaft the axis of rotation of which coincides with that of the runner. This pump-turbine also includes a distributor, which is formed by a set of guide vanes arranged about the runner, and a pre-distributor, arranged between the volute and the distributor and which is formed by a set of stay vanes. In turbine mode, the distributor is arranged downstream of the pre-distributor.
The layout of the first generations of pump-turbines, e.g. pump-turbines several decades old, is strongly influenced by that of the pure Francis type turbines.
During refurbishing, it is common to change the guide vanes without physically changing the stay vanes since the stay vanes contribute to the mechanical strength of the volute and any modification of these would be tricky to implement. This change of guide vanes consists primarily in modifying their camber. The stay vanes then become too radial with respect to the flow from the guide vanes in the pump direction. It is known that this lack of incidence causes additional pressure losses adverse to the good performance of the machine.
One solution therefore consists in reducing the interior geometric angle of the stay vanes, i.e. to make it less radial. This operation would be achievable using techniques of grinding or oversizing. However, modifying the structure of the pre-distributor would risk weakening the mechanical strength of the assembly and this operation would prove difficult given the space requirement of the parts within the distributor.
In addition, the pre-distributor and the flange rings rigidly connected to the pre-distributor are mechanical parts embedded in concrete since they are key components in the mechanical dimensioning of the machine. This is because they are subject to high levels of mechanical stress. A modification in the profile of the stay vanes would therefore lead to a weakening of the structure. In addition, such an operation would be lengthy to implement and very expensive.